The invention relates to a transmission system for transmitting signals between at least two transceiver stations which comprise at least one sub-band coding arrangement and at least one sub-band echo canceller, wherein the sub-band echo canceller is provided to be coupled to a sub-band coding arrangement which coding arrangement comprises at least one synthesis filter bank and at least one analysis filter bank, the synthesis filter bank and the analysis filter bank being provided for the sub-band coding arrangement and also for the sub-band echo canceller.
The invention further relates to an echo canceller and a subscriber terminal unit which has an echo path formed by at least one microphone and at least one loudspeaker and comprises such an echo canceller, as well as a transmission system for transmitting signals between at least two transceiver stations which comprise at least one sub-band coding arrangement and at least such an echo canceller.
Sub-band echo cancellers of this type which will hereinafter be designated echo cancellers for simplicity are necessary, especially, for electrical echo cancellation in speech signals for telephone connections via satellite links and for acoustic echo cancellation in subscriber terminal units, for example, in hands-free facilities of video conference units. In these units there is an additional delay of the speech signal due to intricate and expensive picture encoding.
From "Advances in Speech Signal Processing", Sadaoki Furui, M. Mohan Sondhi, pp. 327 to 356 an echo canceller is known to comprise a canceller that can be switched between a receive path and a send path. The input and output signals of an echo path are then sent through identical analysis filter banks which produce vectors of N sub-band signals which are scanned at a reduced scanning rate. The canceller forms a vector of sub-band signals which correspond to the sub-band echo signals. The resulting sub-band errors at the output of a subtracter arrangement are sent through a synthesis filter bank to produce a full-band signal which is returned to the remote speaker.